Polyesters, such as poly(ethylene terephthalate) or PET, and poly(butylene terephthalate) or PBT, are widely used in the preparation of articles by forming methods such as injection molding and tube extrusion. Many of their properties, including chemical stability, solvent resistance and low permeability to gases, make them attractive candidates for such forming operations as blow molding, profile extrusion and thermoforming.
It is also well known in the art to blend polyesters with other resins to provide other useful properties. Polybutylene terephthalate is commercially blended with aromatic polycarbonates and other ingredients, such as rubbers, for applications requiring high toughness and ductility, combined with resistance to deformation under load at elevated temperatures.
However, it has been found that during the preparation of the polyesters or during the processing of blends of polyesters and polycarbonates at the required high temperatures, chemical interactions, such as ester-carbonate interchange reactions, occur resulting in reduced strength and increased mold cycle times. These chemical interactions are promoted by the activity of the residual titanium ester-based polyesterification catalyst employed during the synthesis of the polyester.
Previous attempts in the prior to overcome these shortcomings have included employing chelate catalysts during the preparation of the polyesters.
For example, McCready, U.S. Pat. No. 4,452,969 teaches using a titanium-containing chelate prepared by the reaction of a tetraalkyl titanate with a substituted carboxylic acid derivative such as methyl salicylate, malic acid, glycine or dibutyl tartrate. Similarly, Brunelle, U.S. Pat. No. 4,452,970 teaches employing a titanium-containing chelate catalyst prepared by reacting a tetraalkyl titanate with an ortho-substituted hydroxyaromatic compound.
Other attempts in the prior art have included adding stabilizers after the esterification reaction stages. For example, Saiki et al., U.S. Pat. No. 4,031,165 teaches employing a titanium catalyst to prepare an aliphatic polyester/aromatic polyester block copolymer and then adding certain phosphorus compound deactivators to prevent random copolymerization.
Bier et al., U.S. Pat. No. 4,115,371; and Horlbeck et al., U.S. Pat. No. 4,208,527 teach reducing the discoloration in polyesters by employing phosphorus containing deactivators after esterification.
Kuhnrich et al., U.S. Pat. No. 4,482,700 teaches preparing polyesters with the use of titanium catalyst and adjuvants prepared from phosphorous acid and monoepoxides to produce polyesters free from discoloration.
Mention is also made of the Sugerman et al. patents, U.S. Pat. Nos. 4,600,789, 4,623,738 and 4,657,988 which teach the use of neoalkoxy organotitanate compounds as coupling agents and repolymerization agents.
While satisfactory results have been achieved with catalyst inactivators, such an additional process step is costly and inefficient from a processing standpoint. It would therefore represent a notable advance in the state of the art if a catalyst could be developed which eliminated this extra additive step.
It has now been found that such a long felt need in the art is satisfied by the polyesterification catalyst systems of the present invention. Unexpectedly, these novel catalysts are capable of catalyzing the polyesterification reaction between the dicarboxylic acid, and/or derivatives thereof, and the glycol, but do not catalyze the interchange reaction between aromatic esters and carbonates.